In optical transmission system, in order to guarantee the precise propagation of signal light, it is necessary to supervise its optical amplifying repeater as well as transmission line through supervisory-signal light. Conventional optical amplifying repeaters capable of being supervised through supervisory signal light are explained below.
FIG. 1 shows a first example of conventional optical amplifying repeaters, which is disclosed in Japanese patent application laid-open No. 6-204949 (1994), capable of being supervised through supervisory signal light from an end-office system. FIG. 2 shows the transfer operation of supervisory signal light in the optical amplifying repeater in FIG. 1.
In the optical amplifying repeater 1, an optical amplifier 2a and a 2.times.2 optical coupler 3a, which means a two-Input and two-output optical coupler, are disposed from the input side to the output side of an up optical fiber transmission line 9a. Also, an optical amplifier 2b and a 2.times.2 optical coupler 3b are disposed from the input side to the output side of an up optical fiber transmission line 9b. Furthermore, up and down optical attenuators 4a, 4b are disposed, in a loop form, between the optical couplers 3a and 3b.
With this composition, a case that main signal light with four wavelengths (.lambda..sub.1 to .lambda..sub.4) multiplexed and supervisory signal light with another wavelength (.lambda..sub.sv) as shown by the spectra of signal light in FIG. 2, where the vertical axis indicates optical intensity and the horizontal axis indicates optical wavelength, are transmitted from an end-office system (not shown) to the up optical fiber transmission line 9a is explained below.
The main signal light and supervisory signal light are amplified by the optical amplifier 2a, and then divided by the optical coupler 3a. Then, the divided main signal light .lambda..sub.1 to .lambda..sub.4 and supervisory signal light .lambda..sub.sv are attenuated by the attenuator 4a, coupled with signal light transmitted through the down optical fiber transmission line 9b by the optical coupler 3b, then returning to the end-office system that transmitted the main signal light .lambda..sub.1 to .lambda..sub.4 and supervisory signal light .lambda..sub.sv. Therefore, the transmission line supervising system of the end-office system can monitor the optical amplifying repeater 1.
FIG. 3 shows a second example of conventional optical amplifying repeaters, which is disclosed in Japanese patent application laid-open No.8-181656 (1996), capable of being supervised through supervisory signal light from an end-office system. FIG. 4 shows the transfer operation of supervisory signal light in the optical amplifying repeater in FIG. 3.
In the optical amplifying repeater 11, an optical amplifier 12a and a 2.times.2 optical coupler 13a are disposed from the input side to the output side of an up optical fiber transmission line 19a. Also, an optical amplifier 12b and a 2.times.2 optical coupler 13b are disposed from the input side to the output side of an up optical fiber transmission line 19b. Furthermore, up and down optical bandpass filters 4a, 4b are disposed, in a loop form, between the optical couplers 3a and 3b.
With this composition, a case that main signal light with four wavelengths (.lambda..sub.1 to .lambda..sub.4) multiplexed and supervisory signal light with another wavelength (.lambda..sub.sv) as shown by the spectra of signal light in FIG. 4, where the vertical axis indicates optical intensity and the horizontal axis indicates optical wavelength, are transmitted from an end-office system (notshown) to the up optical fiber transmission line 19a is explained below.
The main signal light and supervisory signal light are amplified by the optical amplifier 12a, and then divided by the optical coupler 13a. Then, of the divided main signal light .lambda..sub.1 to .lambda..sub.4 and supervisory signal light .lambda..sub.sv, only the supervisory signal light .lambda..sub.sv transmitted at a permittivity shown in FIG. 4 by the optical bandpass filter 14a, coupled with signal light transmitted through the down optical fiber transmission line 19b by the optical coupler 13b, then returning to the end-office system that transmitted the main signal light .lambda..sub.1 to .lambda..sub.4 and supervisory signal light .lambda..sub.sv. Therefore, the transmission line supervising system of the end-office system can monitor the optical amplifying repeater 11.
However, in the first conventional optical amplifying repeater shown in FIG. 1, since the main signal light has to be transferred to the opposite line with the supervisory signal light, there is a problem that the main signal light of the opposite line deteriorates and reduces in its level due to the interference between the main signal light to be transferred and another main signal light to be transmitted through the opposite line, and due to the mixing of the main signal light transferred. Also, when the loss of the return path is set to be large so as to avoid such deterioration of main signal light, the level of supervisory signal light to return reduces. In this case, a high-precision receiving circuit is required for the transmission line supervising system.
On the other hand, in the second conventional optical amplifying repeater shown in FIG. 3, though the supervisory signal light is transferred to the opposite line while being extracted by the optical filter, since it is difficult to narrow the pass band of the optical filter, the main signal light adjacent thereto can leak from the optical filter and is transferred to the opposite line. Therefore, there occurs a problem that the main signal light of the opposite line deteriorates and reduces in its level due to the interference between the main signal light to be transferred and another main signal light to be transmitted through the opposite line, and due to the mixing of the main signal light transferred. Also, when the wavelength of supervisory signal light is located apart from that of main signal light so as to avoid such deterioration of main signal light, the amplifying wavelength band of the optical amplifier needs to be widened.